The present invention relates to a frequency-conversion circuit comprising a multistage frequency-converting means and an oscillator output coupled to at least one oscillator input of the multistage frequency-converting means.
The present invention also relates to a telecommunication system, a receiver, a transmitter, a transceiver, an integrated circuit, and a telephone device provided with such a frequency-conversion circuit.
Such a frequency-conversion circuit is known from "High Integration CMOS RF Transceivers", Proc. of the Workshop on Advances in Analog Circuit Design, Apr. 2-3-4, 1996, Lausanne-Ouchy, Switzerland, by F. Brianti et al., 14 pages. This article describes in particular integrated digital radio architectures having a frequency up or down converting means usable, for example, in low-IF, such as zero-IF, or wideband-IF configurations. By definition, a low-IF configuration, such as a receiver, transmitter or transceiver, is a configuration, wherein the intermediate frequency (IF) is close to zero, or zero in the case of zero-IF. Some known advantages of zero-IF configurations are the high degree of integration on a chip that can be achieved as compared to a heterodyne or conventional IF configuration, because of the possibility to apply filters, such as low-pass filters for channel selectivity, that can be integrated at low cost. Reduced power dissipation, fewer interference problems and better crosstalk control than in a conventional configuration that requires external components can be achieved with a careful design using this zero-IF architecture. The frequency conversion is implemented by means of two stages of mixers (see FIG. 2) resulting in a multistage frequency conversion in order to reduce the requirements imposed upon image rejection filters in front of the mixers as compared to conventional architectures. The mixer stages are quadrature mixer stages each having an I-path and a Q-path in order to properly distinguish between positive and negative frequencies, corresponding to upper and lower sidebands of an input RF signal. These quadrature mixers eliminate an off-chip IF filter function. Each of the two stages is being fed by a separate voltage-controlled oscillator (VCO) phase-locked loop (PLL) synthesizer circuit. Both synthesizer circuits are connected to a common crystal oscillator. The problem of the known frequency conversion circuit is that two synthesizer circuits are needed, which circuits lead to significant costs and require a large surface, area and much power.